Variable flow rate valves as well as two position on-off valves, such as slidably-mounted sleeve valves, play an essential part in optimized well management in oil wells of recent design. It is thus important for them to offer good reliability so that they can operate without maintenance for several years. Any maintenance on such valves is costly (removal and re-insertion of the production tubing), and it results being interrupted, which goes against the object that they are supposed to achieve (optimized well profitability).
One of the essential problems lies in the need to provide dynamic sealing gaskets on the production tubing, on either side of the holes formed therein, so that the valve is properly closed when the closure sleeve occupies the corresponding position.
Such dynamic sealing gaskets are inevitably made of a relatively soft material such as an elastomer or plastic. They are thus very fragile. In particular, they are very sensitive to wear, to abrasion, and to fatigue, and they are very poor at withstanding the flow of the petroleum fluid.
An additional problem appears when the valve is opened after being closed for a certain amount of time. There is then a pressure difference that is sometimes large between the dynamic pressure inside the production tubing and the higher or lower static pressure outside the tubing in the underground reservoir being tapped. On valve opening, the pressure equalization that tends to occur between the outside and the inside (or inside to outside) of the production tubing immediately imparts a high flow rate to the petroleum fluid. The high-rate flow sweeps the surface of the sealing gasket. If no particular precaution is taken, the gasket is then torn away or else it wears very rapidly.
In an attempt to remedy that drawback, it is common to limit the rate of the flow reaching the sealing gasket in question by interposing rings (generally made of metal or of polytetrafluoroethylene) between the gasket and the holes provided in the production tubing. However, such rings are not very effective, and they do not prevent the gasket from suffering accelerated damage as a result of the valve being opened.